Microwave study of the critical state in high-Tc superconductors

New experimental features of the field-modulated microwave absorption in high-Tc ceramic superconductors are presented. The proposed model for the loss mechanism can explain the complicated field and temperature dependences of the absorption signals.

https://iopscience.iop.org/article/10.1209/0295-5075/10/6/015/pdf

Mechanisms of Field-Dependent Microwave Absorption in High-Tc Ceramic Superconductors

Review of stability in high temperature superconductors with emphasis on flux jumping

Paramagnetic Meissner effect and related dynamical phenomena

We have experimentally studied the magnetic shielding properties of a cylindrical shell of BiPbSrCaCuO subjected to low frequency AC axial magnetic fields. The magnetic response has been investigated as a function of the dimensions of the tube, the magnitude of the applied field and the frequency. These results are explained quantitatively by employing the method of Brandt (1998 Phys. Rev. B 58 6506) with a Jc(B) law appropriate for a polycrystalline material. Specifically, we observe that the applied field can sweep into the central region either through the thickness of the shield or through the opening ends, the latter mechanism being suppressed for long tubes. For the first time, we systematically detail the spatial variation of the shielding factor (the ratio of the applied field over the internal magnetic field) along the axis of a high-temperature superconducting tube. The shielding factor is shown to be constant in a region around the centre of the tube, and to decrease as an exponential in the vicinity of the ends. This spatial dependence comes from the competition between two mechanisms of field penetration. The frequency dependence of the shielding factor is also discussed and shown to follow a power law arising from the finite creep exponent n.

Magnetic shielding properties of high-temperature superconducting tubes subjected to axial fields

Microwave absorption in a dc magnetic field up to 12 kG, attributed to nonequilibrium contributions to the ac susceptibility, appears at ${T}_{c}$ as the sample is cooled. ESR measurements of Y-Ba-Cu-O show that ${\mathrm{Cu}}^{2+}$ exists only in the fraction of the sample which is not superconducting in a distorted octahedral surrounding.

/pdf/microwave-absorption-in-the-superconducting-and-normal-3ti9qt5klc.pdf

Microwave absorption in the superconducting and normal phases of Y-Ba-Cu-O

Period doubling in a perturbed sine-Gordon system, a long Josephson junction

High‐Tc superconducting tubes have been developed for magnetic shielding of SQUIDs at 77 K. The characteristics of such tubes show that adequate shielding exists for magnetic fields up to a critical field determined by the current density of the tube. Relaxation phenomena with two different time dependencies are observed when the external magnetic field enters the tubes.

Magnetic shielding and relaxation characteristics of superconducting YBa2Cu3O7 tubes

Simulations of long Josephson junctions in an external magnetic field show a variety of chaotic behaviors. They consist of period-doubling bifurcation, intermittency between and within Fiske steps, and quasiperiodic motion at Fiske steps. A physical explanation for the chaotic behavior is presented. The influence of surface resistance to the chaotic motion is investigated. A diagram in terms of dissipation and magnetic field is constructed to show where chaos occurs for a long junction.

Chaos in long Josephson junctions without external rf driving force

The large magnetic‐field‐dependent ac absorption in superconducting Y1 Ba2 Cu3 Oy ceramics and powders decreases slowly with increasing frequency in the range 2–16 MHz. The magnetic‐field‐dependent ac absorption is observed below Tc in superconducting Y1 Ba2 Cu3 Oy ceramics, powders and twinned crystals and in (La,Sr)2 CuO4 and BaPb0.75 Bi 0.25 CuO3 powders and appears to contain distinct contributions from trapped flux and/or intragrain tunnel junctions and from ordinary superconductor surface impedance. The contribution from trapped flux and/or intragrain tunnel junctions is important in dc magnetic fields of up to approximately 20 G at all temperatures below Tc. There is an identifiable modulated ac absorption which changes slowly over more than 12 kG at low temperatures and which we propose is primarily caused by the ac surface impedance with possible contributions from the critical state. A twinned crystal of Y1 Ba2 Cu3 Oy exhibits the same ac absorption except that it has an anisotropic dependence o...

/pdf/absorption-at-radio-frequencies-in-superconducting-3i91blcz9p.pdf

D.J. Zheng

Papers

Microwave absorption in the superconducting and normal phases of Y-Ba-Cu-O

Period doubling in a perturbed sine-Gordon system, a long Josephson junction

Magnetic shielding and relaxation characteristics of superconducting YBa2Cu3O7 tubes

Chaos in long Josephson junctions without external rf driving force

Absorption at radio frequencies in superconducting Y1Ba2Cu3Oy